A conventional control system for a rotary machine performs current feedback control, in which command voltages of the d-axis and the q-axis as operation amounts for feedback-controlling currents of the d-axis and the q-axis to command currents. Specifically, each of the command voltages is calculated as a sum of a proportional term and an integral term, which inputs a difference between a current flowing in a rotary machine and a command current. An optimum value of each gain of the proportional term and the integral term varies with a resistance and an inductance in a voltage equation, which defines a relation between a current and a voltage of the d-axis and the q-axis (non-patent document 1). The inductance of the rotary machine varies with a current flowing in the rotary machine. The gain is therefore variably set in accordance with an operating condition of the rotary machine.
Model estimation control is also proposed recently as a replacement of the above-described current feedback control. According to the model estimation control, a plurality of switching modes is provisionally set in correspondence to on/off operations of switching devices of an inverter. A current is estimated for each of the switching modes and an optimum switching mode is selected based on the estimated current. The current is estimated by using the voltage equation, which defines the relation between the current and the voltage of the d-axis and the q-axis.
[Patent document 1] JP 2008-228419A
[Non-patent document 1] “Theory and Design of an AC Servo System” authored by Hidehiko Sugimoto, published by Sougou Denshi Shuppan Sha, pages 72-82.
It was found by the inventor that, in performing the model estimation control, the accuracy of estimation of current degrades as the current flowing in the rotary machine flows. It was also found that the degradation of the accuracy of estimation cannot be countered sufficiently as in the case of the current feedback control even if the inductance in the voltage equation is varied.